1. Field of the Invention
The present invention relates generally to laser drilling, and more particularly to a method and apparatus for drilling a hole with a laser while avoiding exposure of nearby articles to the laser beam after the hole has been drilled.
2. Description of the Related Art
Lasers have proven very effective in drilling holes in components, such as in the aerospace industry. For example, lasers are commonly used to drill air cooling holes in turbine airfoils such as blades and vanes. A pulsed laser beam is directed at the article, and molten material from the focus region of the laser beam is expelled from the article.
According to one known method, an Nd:YAG laser generates pulses having a pulse energy of 10 joules, a pulse duration of 1 millisecond, and a pulse repetition rate of 10 Hz, to drill holes in hollow turbine blades. The pulses are generated in a "free running" mode in which the lamps are pulsed at 10 Hz to produce the pulsed laser beam. For a hollow turbine blade having a wall thickness of about 1/8 inch, it typically requires 3 or 4 pulses to break through to the interior of the turbine blade. It is also common to apply several "clean up" laser pulses to the hole after the laser has broken through, to refine the geometry of the drilled hole.
Once the laser has broken through to the interior of the airfoil, however, the laser beam may cause serious damage to the wall of the airfoil opposite the hole, especially if additional clean up pulses are applied to the hole to refine its shape. To protect the opposite wall of the airfoil from being damaged by the laser, known methods utilize wax as a "beam blocker". As shown in FIG. 1, the airfoil 10 is filled with wax 12 before drilling so that the laser beam 14 will be absorbed by the wax 12 to prevent damage to the opposite wall 16.
It is very labor intensive, however, to fill the airfoil with wax before drilling and to remove the wax once the hole has been drilled. The wax is typically heated and injected with a syringe, and removal, which typically involves high pressure steam cleaning, generates contaminated waste water which must be disposed of appropriately. A residual layer of wax left on the interior of the airfoil may also complicate subsequent application of a coating on the interior of the airfoil. The wax may, in addition, affect the interaction between the laser beam and the article being drilled, which introduces a variability into the drilling process, adversely affecting precision.
It would be desirable, therefore, to have a method and apparatus for drilling a hole through a hollow airfoil, for example, while avoiding damage to a wall opposite the hole being drilled, without encountering the many disadvantages involved in using a wax beam blocker inside the airfoil.